The invention relates to a method and arrangement for storing on tape information produced by a switching exchange system. The requirements for storing data in exchange systems vary widely. Charging data and different traffic measurement reports are stored by magnetic tape recorders. In addition, backup copies of disk files are usually stored on magnetic tape. It is worth storing data on magnetic tape if larger amounts are to be stored into a portable permanent storage.
Charging data are produced as a switching exchange switches calls. Charging data is stored on magnetic tape almost in real time, and thus the amount of data to be stored at a time may be small. In addition, data storage is periodic and non-predefined, as the frequency for storing data depends on the number of calls.
The requirements set by switching exchange systems on storage of charging information are continuously increasing. The required traffic volumes grow. In addition, the trend is increasingly toward call-by-call charging, in regard to which records to be stored on tape are produced from each call. The amount of data to be stored is so large that the capacities of conventional start/stop-type recorders currently used in storing charging information are insufficient with high traffic. On the other hand, streamer-type recorders are too slow with low traffic.
Magnetic recorders are classified in two categories based on the storage type: start/stop-type recorders and streamer-type recorders. Start/stop recorders start tape movement always when the recorder receives a data block to be stored and stop the tape after storing the data block. Streamer-type recorders do not stop the tape between data blocks if a new block reaches the recorder soon enough.
Start/stop recorders are faster than streamer recorders when data are stored in relatively small amounts at a time, as in start/stop recorders stopping and starting the tape is mechanically realized significantly faster than in streamer recorders. If data can be stored on tape as a continuous data stream so that the recorder does not have to stop the tape before a new data block to be stored reaches the recorder, a streamer recorder is faster. In this case, storage rate is about equal to that of hard disks. In addition, the storage capacity of streamer recorders is usually considerably higher, around several gigabytes, than that of start/stop recorders, which is below 100 megabytes. If a streamer recorder is used for storing small amounts of data so that the recorder stops the tape before the next data to be stored reaches the recorder, one storage may, depending on the recorder, require even 15 seconds, if the length of the data to be stored is less than 1 kB.
The total storage capacity of streamer recorders decreases the more frequently the tape has to be stopped during storage. During stop and start the tape moves considerably before the speed of the tape is adequate for storage or until the tape has stopped. This part of the tape cannot be used for storage of useful data. The mechanics of start/stop recorders prevent a corresponding long interval, where no useful data can be stored, from being created on the tape.